Animations can be unique to a single Skeletal Mesh or they can be reused for any number of Skeletal Meshes as long as each Skeletal Mesh uses the same skeleton. All you really need to create an animation and export it into Unreal using the FBX pipeline is a skeleton that has been animated. Having a mesh bound to the skeleton is completely optional, though it makes the process of creating the animation much easier as you can see how the mesh is deforming during the animation. At export time, only the skeleton is required though.